1. Field of the Invention
The present invention relates to the detection of a disconnection in an optical transmission line. More particularly, the present invention relates to the use of Brillouin scattering to detect a disconnection in an optical transmission line.
2. Description of the Related Art
With the progression of optical communication systems, optical output power of optical transmission apparatuses is increasing as a result of, for example, an increase of transmission distance.
Accordingly, if a worker erroneously cuts an optical fiber or, for example, opens an optical connector while light is being transmitted, the light may radiate from the cut fiber or opened connector at a dangerously high intensity. Therefore, it is being requested that optical transmission apparatuses provide a monitoring mechanism to suspend or reduce optical output by detecting disconnection of the transmission line due to cutting of an optical fiber or opening of an optical connector or similar device.
Standards such as IEC (International Electrotechnical Commission) and JIS (Japanese Industrial Standard) specify regulations to be observed by a monitoring mechanism in order to attain the safety of workers for maintenance and repair when an optical fiber is disconnected. The regulations relate to, for example, detection of a failure, transition to a failure condition, halting/reducing pump light or intermitted generation of pumped light upon detection of a failure, and recovery after the failure.
Fresnel reflection is conventionally used by a monitoring mechanism in an optical transmission apparatus to detect disconnection of the transmission line.
For example, FIG. 1 is a diagram illustrating a conventional optical transmission apparatus having a monitoring mechanism for detecting disconnection of a transmission line, and which is based on Fresnel reflection. An optical transmission apparatus using Fresnel reflection, such as that in FIG. 1, can be understood from Japanese Laid-Open Publication No. 3-034529.
Referring now to FIG. 1, an optical transmission apparatus 1B comprises an optical amplifying/controlling section 4A and a Fresnel reflection light detecting section 5A. Optical amplifying/controlling section 4A comprises an optical amplifying section 17A, an optical variable attenuating section 16A, a coupler 19C, a photodiode (PD) 11B, a signal detecting section 15B and a control section (CTRL) 18A. Moreover, light detecting section 5A comprises a coupler 19D, a photodiode (PD) 11D, and a signal detecting section 15D. Intensity of the light output from optical amplifying/controlling section 4A can be varied by controlling gain of optical amplifying section 17A or attenuation of optical variable attenuating section 16A.
When an optical fiber of the transmission line is broken or an optical connector is opened, the core of optical fiber is exposed to the air and change is thereby occurred in refractive index of medium in which the light is transmitted. Accordingly, reflection of light (Fresnel reflection) is generated. When disconnection such as break of an optical fiber is generated in the transmission line to which the light is output from optical transmission apparatus 1B, signal light transmitted from optical amplifying/controlling section 4A and output light, such as monitoring control light, are partly reflected at the disconnection point with the Fresnel reflection and are then returned to optical transmission apparatus 1B.
The returning light on the transmission line output from optical transmission apparatus 1B is partly branched with coupler 19D of light detecting section 5A, converted to an electrical signal with photodiode 11D and signal detecting section 15D, and is then transferred to control section 18A.
If disconnection is generated in the transmission line, intensity of the returning light to optical transmission apparatus 1B increases due to the Fresnel reflection. Therefore, disconnection of the transmission line can be determined by detecting that intensity of the returning light exceeds a breaking threshold value. When disconnection of the transmission line is determined, optical transmission apparatus 1B maintains an output thereof within a safety standard (hereinafter referred to as a “safe light condition”) through reduction of the gain of optical amplifying section 17A or increase of attenuation of optical variable attenuating section 16A.
If the transmission line is in the disconnected condition, Fresnel reflection is also generated due to the light output within a range of the safety standard. Therefore, disconnection and recovery of transmission line can be determined even within the safe light condition. When disconnection of the transmission line is recovered, the returning light to optical transmission apparatus 1B due to the Fresnel reflection disappears. Accordingly, recovery of disconnection of the transmission line can be determined by detecting that intensity of the returning light is reduced to less than a certain breaking threshold value. When recovery of disconnection of the transmission line is determined, optical transmission apparatus 1B returns to the operation of providing ordinary output by resetting the gain of optical amplifying section 17A and attenuation of optical variable attenuating section 16A.
The above-described manner of detecting disconnection of the transmission line is based on the detection of Fresnel reflection at the disconnection point. However, intensity of the returning light due to the Fresnel reflection varies depending on conditions of the disconnection point. For example, amount of returning light due to the Fresnel reflection generated when an APC (Angled Physical Contact) connector formed in the structure to reduce the returning light by forming angled polishing surface is opened becomes smaller than the amount of the returning light due to the Fresnel reflection generated when a different type of connector (for example, a PC (Physical Contact)) connector is opened.
Moreover, even when an optical fiber is broken, the amount of returning light due to the Fresnel reflection is different to a large extent depending on the condition of the cutting surface. Accordingly, it is also probable that disconnection of the transmission line cannot be detected only with detection of increase in amount of the returning light due to the Fresnel reflection.
Further, since intensity of the light transmitted to the transmission line in the safe light condition is rather low, if amount of the returning light due to the Fresnel reflection is small, it becomes difficult, in some cases, to determine the recovery of disconnection due to disappearance of the Fresnel reflection even when the safe light condition can be set through detection of the Fresnel reflection.
There are other mechanisms for detecting a disconnection of the transmission line. For example, a monitoring control signal can be transmitted to an optical transmission apparatus from a downstream apparatus in the reverse direction of the transmission line. In accordance with detection of the monitoring control signal by the optical transmission apparatus, an output of signal light from the optical transmission apparatus can be kept within the safety standard by detecting disconnection of the transmission line in the downstream side. However, with this type of monitoring mechanism, it is impossible to directly perform control such as suspension of output and stoppage of output of the light transmitted from the down-stream station.
Other manners of detecting disconnection of the transmission line include using information obtained from an opposite transmission line to detect disconnection.